Scotland Yard said the arrests in Luton, about 56 kilometres north of London, were part of a "pro-active" investigation.
In a statement, Scotland Yard said: "Four men have been arrested in Luton this morning on suspicion of committing offences under the Terrorism Act.
"The arrests were carried out in a joint operation by the Met's counter terrorism command (SO15) and the eastern counter terrorism and intelligence unit (ECTIU).
"The men who are all in their 30s have been arrested on suspicion of being involved in the commission, preparation and instigation of acts of terrorism. They have been taken into custody at a London police station."
After the arrests, seven addresses and several vehicles are being searched in Luton, Metropolitan Police said. The arrests are not connected to last month's attacks in Paris, it added.
"Today's activity is part of an ongoing proactive investigation concerning individuals in the Luton area. It is not connected to the recent attacks in Paris," the statement added.
A decision to detain suspects can be made when counter- terrorism investigators believe they have enough material to stage arrests.
The attacks in Paris last month which left 130 dead have increased anxiety among UK officials of an attack on British soil. Britain has increased its threat level from terrorism to "severe" -- its second highest-- which means an attack is "highly likely".